Cellulosic fibrous structures, such as paper towels, facial tissues, and toilet tissues, are a staple of every day life. The large demand and constant usage for such consumer products has created a demand for improved versions of these products and, likewise, improvement in the methods of their manufacture. Such cellulosic fibrous structures are manufactured by depositing an aqueous slurry from a headbox onto a Fourdrinier wire or a twin wire paper machine. Either such forming wire is an endless belt through which initial dewatering occurs and fiber rearrangement takes place.
After the initial formation of the web, which becomes the cellulosic fibrous structure, the papermaking machine transports the web to the dry end of the papermaking machine. In the dry end of a conventional papermaking machine, a press felt compacts the web into a single region cellulosic fibrous structure prior to final drying. The final drying is usually accomplished by a heated drum, such as a Yankee drying drum.
One of the significant aforementioned improvements to the manufacturing process, which yields a significant improvement in the resulting consumer products, is the use of through-air drying to replace conventional press felt dewatering. In through-air dating, like press felt drying the web begins on a forming wire, which receives an aqueous slurry of less than one percent consistency from a headbox. Typically, initial dewatering takes place on the forming wire. The forming wire is not typically exposed to web consistencies of greater than 30 percent. From the forming wire, the web is transferred to an air pervious through-air-drying belt.
Air passes through the web and the through-air-drying belt to continue the dewatering process. The air passing the through-air-drying belt and, the web is driven by vacuum transfer slots, other vacuum boxes or shoes, prelayer rolls, etc., and molds the web to the topography of the through-air-drying belt, increasing the consistency of the web. Such molding creates a more three-dimensional web, but also causes pinholes, if the fibers are deflected so far in the third dimension that a breach in fiber continuity occurs.
The web is then transported to the final drying stage where the web is also imprinted. At the final drying stage, the through-air drying belt transfers the web to a heated drum, such as a Yankee drying drum for final drying. During this transfer, portions of the web are densified during imprinting, to yield a multi-region structure. Many such multi-region structures have been widely accepted as preferred consumer products. An example of an early through-air-drying belt which achieved great commercial success is described in commonly assigned U.S. Pat. No. 3,301,746, issued Jan. 31, 1967 to Sanford et al.
Over time, further improvements became necessary. A significant improvement in through-air-drying belts is the use of a resinous framework on a reinforcing structure. This arrangement allows drying belts to impart, continuous patterns, or, patterns in any desired form, rather than only the discrete patterns achievable by the woven belts of the prior art. Examples of such belts and the cellulosic fibrous structures made thereby can be found in commonly assigned U.S. Pat. Nos. 4,514,345, issued Apr. 30, 1985 to Johnson et al.; 4,528,239, issued Jul. 9, 1985 to Trokhan; 4,529,480, issued Jul. 16, 1985 to Trokhan; and 4,637,859, issued Jan. 20, 1987 to Trokhan. The foregoing four patents are incorporated herein by reference for the purpose of showing preferred constructions of patterned resinous framework and reinforcing type through-air-drying belts, and the products made thereon. Such belts have been used to produce extremely commercially successful products such as Bounty paper towels and Charmin Ultra toilet tissue, both produced and sold by the instant assignee.
As noted above, such through-air-drying belts used a reinforcing element to stabilize the resin. The reinforcing element also controlled the deflection of the papermaking fibers resulting; from vacuum applied to the backside of the belt and airflow through the belt. The early belts of this type used a fine mesh reinforcing element, typical having approximately fifty machine direction and fifty cross-machine direction yarns per inch. While such a fine mesh was acceptable from the standpoint of controlling fiber deflection into the belt, it was unable to stand the environment of a typical papermaking machine. For example, such a belt was so flexible that destructive folds and creases often occurred. The fine yarns did not provide adequate seam strength and would often burn at the high temperatures encountered in papermaking.
Yet other drawbacks were noted in the early embodiments of this type of through-air-drying belt. For example, the continuous pattern used to produce the consumer preferred product did not allow leakage through the backside of the belt. In fact, such leakage was minimized by the necessity to securely lock the resinous pattern onto the reinforcing structure. Unfortunately, when the lock-on of the resin to the reinforcing structure was maximized, the short rise time over which the differential pressure was applied to an individual region of fibers during the application of vacuum often pulled the fibers through the reinforcing element, resulting in process hygiene problems and product acceptance problems, such as pinholes.
A new generation of patterned resinous framework and reinforcing structure through-air-drying belts addressed some of these issues. This generation utilized a dual layer reinforcing structure having vertically stacked machine direction yarns. A single cross-machine direction yarn system tied the two machine direction yarns together.
For paper toweling, a coarser mesh, such as thirty-five machine direction yarns and thirty cross-machine direction yarns per inch, dual layer design significantly improved the seam strength and creasing problems. The dual layer design also allowed some backside leakage to occur. Such allowance was caused by using less precure energy in joining to the resin to the reinforcing structure, resulting in a compromise between the desired backside leakage and the ability to lock the resin onto the reinforcing structure.
Later designs used an opaque backside filament in the stacked machine direction yarn dual layer design, allowing for higher precure energy and better lock-on of the resin to the reinforcing structure, while maintaining adequate backside leakage. This design effectively decoupled the tradeoff between adequate resin lock-on and adequate backside leakage in the prior art. Examples of such improvements in this type of belt are illustrated by commonly assigned U.S. patent application Ser. No. 07/872,470 filed Jun. 15, 1992 U.S. Pat. No. 5,334,289 in the names of Trokhan et al., Issue Batch No. V73. Yet other ways to obtain a backside texture are illustrated by commonly assigned U.S. Pat. Nos. 5,098,522, issued Mar. 24, 1992 to Smurkoski et al.; 5,260,171, issued Nov. 9, 1993 to Smurkoski et al.; and 5,275,700, issued Jan. 4, 1994; to Trokhan, which patents and application are incorporated herein by reference for the purpose of showing how to obtain a backside texture on a patterned resin and reinforcing structure through-air-drying belt.
As such resinous framework and reinforcing structure belts were used to make tissue, such as the commercially successful Charmin Ultra noted above, new issues arose. For example, one problem in tissue making is the formation of small pinholes in the deflected areas of the web. Pinholes are strongly related to the depth that the web deflects into the belt. The depth comprises both the thickness of the resin on the reinforcing structure, and any pockets within the reinforcing structure that permits the fibers to deflect beyond the imaginary top surface plane of the reinforcing structure. Typical stacked machine direction yarn dual layer reinforcing structure designs have a variety of depths resulting from the particular weave configuration. The deeper the depth within a particular location of the weave that is registered with a deflection conduit in the resin, the greater the proclivity for a pinhole to occur in that area.
Recent work according to the present invention has shown that the use of triple layer reinforcing structures unexpectedly reduces occurrences of pinholes. Triple layer reinforcing structures comprise two completely independent woven elements, each having its own particular machine direction and cross-machine direction mesh. The two independent woven elements are typically linked together with tie yarns.
More particularly, the triple layer belt preferably uses a finer mesh square weave as the upper layer, to contact the web and minimize pinholes. The lower layer or machine facing layer utilizes coarser yarns to increase rigidity and improve seam strength. The tie yarns way be machine direction or cross-machine direction yarns specifically added and which were not present in either layer.
Alternatively, the tie yarns may be comprised of cross-machine direction or machine direction tie yarns from the upper and/or lower element of the reinforcing structure. Machine direction yarns are preferred for the tie yarns because of the increased seam strength they provide.
However, this design still does not solve the problem where backside leakage may be required. Reference to the prior art teachings of backside texturing do not solve this problem either. For example, the aforementioned U.S. patent application Ser. No. 07/872,470 U.S. Pat No. 5,334,289 teaches the use of opaque yarns to prevent curing of resin therebelow. The resin that is not cured is washed away during the belt making process and imparts a texture to the backside of the belt. However, such a teaching further states that it is preferable the machine directions yarns be opaque because the machine direction yarns are generally disposed closer to the backside surface of the reinforcing structure than the cross-machine yarns. Such a description is not correct, however, if the machine direction yarns are used as tie yarns.
Thus, the machine direction yarn must serve either one of two mutually exclusive functions: it must either remain within the lower layer to prevent texture from going too deep into the belt, or rise out of the lower layer to tie the lower layer relative to the first layer. Compounding the problem with triple layer belts is any opaque machine direction yarns used as tie yarns will disrupt the lock-on of the resin below because such yarns intermittently are disposed on the topside of the reinforcing structure.
Accordingly, it is an object of this invention to provide a belt which overcomes the tradeoff between high seam strength and minimal pinholing. It is further an object of this invention to provide a belt which overcomes the tradeoffs between backside leakage and low resin lock-on. The prior art has not yet provided a belt which produces consumer desired products (minimal pinholing) with a long lasting belt (high seam strength and high rigidity) and which does not lose functional components during the manufacture of the consumer product (poor resin lock-on).